[sdiy] NIC-based wave folder/shaper

[Ian Fritz]

Friends --

Here is an amazing little circuit I just worked out.  It takes a saw input 
and continuously deforms it from saw to triangle to inverted saw.  It takes 
just one op amp, three fixed resistors and a pot!

The circuit is a simple voltage divider -- with one catch.  The lower leg 
of the divider is a variable *negative* resistor over part of its operating 
range. The negative resistance region folds the waveform.

Here's how to hook it up, if you have a whiteboard and five minutes to spare.

I/O voltages:  Vin to left side of a 47k resistor, Vout to the right side.

Op amp inverting input:  Connect to right side of (above) 47k resistor and 
through a 100k resistor to op amp output.

Op amp non-inverting input: connect through another 100k resistor to op amp 
output.  Also connect through a 50k pot to ground.

That's basically it.  You may also need a small stabilizing cap (10 p) 
across the negative feedback path.  And you may want to buffer the output 
if you use a long output lead.

This is just a first shot at the circuit. You will probably find that the 
ends of the waveform don't quite connect. This can be fixed, if desired, by 
adjusting the fixed resistors slightly.  Additionally, the amplitude 
changes with shape, which could be fixed with a ganged pot controlling a 
variable gain stage.

The circuit could easily be voltage controlled using an OTA or a 
FET-resistor in place of the pot.  A second OTA or FET could manage the 
amplitude leveling.

Fun, fun, fun!

   Ian